1. Field of the invention
The invention relates to one reprocessing of used evaporation boats.
2. Background Art
Evaporation boats are generally used to evaporate metals such as aluminum. Examples thereof are described in the following U.S. Pat. Nos. 4,373,952, 4,089,643, 3,915,900, 3,813,252, 3,803,707, 3,582,611, 3,544,486, 3,256,103, 3,181,968, 2,984,807, 2,962,528.
During the normal operation of evaporation boats, an erosion of the material components of the boat takes place as a result of the interaction with molten aluminum. Although only a small amount of the material components is eroded, the boat loses its serviceability in this way since the effective cross section of the boat and the electrical resistivity are altered. In this case, the boat is normally rejected and replaced after it has usually been worn away by only 10 to 15%.
A method for reprocessing the used evaporators is described in German Offenlegungsschrift 38 39 730 (corresponding to U.S. Pat. No. 4,847,031). According to the method described therein, the used boats, which are as a rule contaminated with fairly large amounts of aluminum, are first treated with hot sodium hydroxide solution in order to remove the aluminum. The boats are then neutralized with hydrochloric acid, washed with water, ground to form a powder and sieved. The sieved powder is used to produce new boats. This method has the following disadvantages:
The three treatment steps for removing aluminum residues from the evaporation boats are very expensive. Fairly large amounts of contaminated hydroxide solutions and acids, and also of solid aluminum-containing waste, are produced. This waste has to be disposed of in an expensive and environmentally polluting way. There is the danger that the material purified by this method is contaminated with alkali metal or OH ions. The latter manifests itself, especially in the case of the oxidation-susceptible, very fine powder, in such a way that the recycled material has higher oxygen values than the original evaporator material. As a result, an evaporator made of recycled material suffers quality losses.
The object of the invention was to provide a method which makes possible an environmentally friendly, low-waste reprocessing of used evaporation boats.